Method and apparatus for assigning spreading codes

ABSTRACT

A method is taught for transmitting message signals in a communication system having a new call entering the communication system and Walsh codes that can be active and inactive. The method includes dividing the Walsh codes into bins and determining the number of active Walsh codes in the bins. Selecting a Walsh code in accordance with the Walsh code number determination and assigning the selected Walsh code to the new call are set forth.

BACKGROUND

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/176,740, filed Oct. 20, 1998, entitled “METHOD AND APPARATUSFOR ASSIGNING SPREADING CODES” and assigned to the assignee of thepresent invention.

I. FIELD

[0002] This invention relates to the field of communications systemsand, in particular, to the field of transmission of message signals in acommunications system.

II. PRIOR ART

[0003] It is well known in the art of cellular communications systems tomix message signals to be transmitted with spreading code vectors suchas Walsh code vectors. This permits the message signals to be combined,transmitted, and then separated from each other at the receiver, aftertransmission. It is possible to separate the received signals becausethe spreading code vectors are orthogonal and they provide a theoreticalinterference of zero between the signals that are combined.

[0004] In order to perform these operations it is known to randomlyassign one of the available spreading codes to each new originating callor each new handoff call added to the communications system. However,random assignment of spreading codes in this manner may result in largepeaks in the transmit power level of the combined signals.

[0005] A serious consequence of the power level peaks is that the poweramplifier that amplifies the combined signals can be temporarily driveninto a nonlinear region and saturated. This can cause interferencebetween the combined signals, particularly between signals on adjacentchannels. The interference between the combined signals can causedegradation of the separated and recovered signals.

[0006] This problem can be solved by providing a power amplifier with anincreased capacity. Such a power amplifier is not driven into itsnonlinear region by the peaks in the power level of the combinedsignals. However, this is an expensive and inefficient solution to theproblem because the increased capacity of the power amplifier is notused during the remaining ninety-nine percent of the time.

[0007] Thus, it is desirable to provide a system and method forsmoothing the transmit power level of the combined signals caused byrandom assignment of spreading codes in order to cause fewer peaks anddrive the power amplifier into its nonlinear region less frequently.

SUMMARY

[0008] A method is taught for transmitting message signals in acommunication system having a new call entering the communication systemand Walsh codes that can be active and inactive. The method includesdividing the Walsh codes into bins and determining the number of activeWalsh codes in the bins. Selecting a Walsh code in accordance with theWalsh code number determination and assigning the selected Walsh code tothe new call are set forth. The Walsh codes have indices and the Walshcodes are divided into bins according to the indices. The Walsh codesare divided into cycles according to the indices and if the number ofbins is n the Walsh codes are divided into bins in accordance with thevalue of their indices modulo n. The minimum number of active Walshcodes in the bins is determined and the Walsh code is selected inaccordance with the minimum number of active codes. A plurality of thebins can contain the minimum number of active Walsh codes. The methodalso sets forth selecting a bin of the plurality of bins containing theminimum number of active Walsh codes and selecting a Walsh code from theselected bin. A subset of the bins containing the minimum number ofactive Walsh codes is selected and a first predetermined bin with apreference lower than a preference for the remaining bins of the subsetof the bins is selected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify corresponding elements throughout and wherein:

[0010]FIG. 1 shows a block diagram representation of a system forgenerating waveforms suitable for transmission in a communicationssystem;

[0011]FIG. 2 shows a block diagram representation of a system forgenerating waveforms suitable for transmission in a communicationssystem in accordance with the present invention;

[0012]FIG. 3 shows a graphical representation of a comparison of thepeak to average ratio of a plurality of different Walsh code sets;

[0013]FIG. 4 shows a graphical representation of a comparison of thepeak to average ratio at equal channel gains and the peak to averageratio at highly unequal channel gains for a plurality of Walsh codesets;

[0014] FIGS. 5A-E show flow chart representation of the biased binbalancing algorithm of the present invention;

[0015]FIG. 6 shows a state diagram representing the states of the methodof the present invention; and

[0016] FIGS. 7-9 show graphical representations of comparisons of theeffects of differing assignments of Walsh code vectors.

DETAILED DESCRIPTION

[0017] A message signal such as a Walsh signal can be represented as avector having the components −1/+1. A corresponding binary spreadingcode such as a Walsh code can be represented as a vector having thecomponents 0/1. A Walsh code vector can be represented as W with asubscript. The subscript is used to represent the Walsh code index ofthe code vector. The ordering of the code index is a standard orderingsuch as W_(i) or W_(i)[n]. The corresponding binary Walsh code of theWalsh signal can be represented by w with a subscript, for example,w_(i). It will be understood by those skilled in the art that w_(i) canbe obtained from W_(i) by replacing each 1 within W_(i) with a 0 andreplacing each -1 within W_(i) with 1.

[0018] The binary Walsh code is a linear code. Thus, if w_(i) and w_(j)are Walsh code vectors then w_(i)+w_(j) modulo 2 is also a binary Walshcode vector. Any code vector in a linear code can be expressed as alinear combination of a smaller set of code vectors referred to as basisvectors. In particular, a binary linear code of size 2^(m) can beexpressed as a linear combination of certain sets of m vectors. Forexample, {w₁, w₂, w₄, w₅, w₁₆, w₃₂} can be selected as the set of basisvectors for a binary Walsh code of size 64 wherein the index is zerobased. This is the type of code specified in the industry standard formobile communications systems, IS-95.

[0019] Thus, any binary Walsh code vector can be represented as:

w _(i) =c ₁ w ₁ +c ₂w₂ +c ₄w₄ +c ₈ w _(8,) +c ₁₆ w ₁₆ +c ₃₂ w ₃₂

[0020] where the addition is modulo 2 and c₁, c₂, c₄, c₈, c₁₆, c₃₂ arebinary scalars that can therefore only take on the values zero or one.Each distinct selection of the binary parameters {c₁, c₂, c₄, c₈, c₁₆,c₃₂} out of a total of sixty-four gives a distinct binary Walsh codevector. Furthermore, the vector set {w₁, w₂, w₄, w₈, w₁₆, w₃₂} is onlyone of several possible choices of basis vectors. Another set of basisvectors can be {w₁, w₃, w₆, w₉, w₁₇, w₃₃}. However, for the purpose ofsimplifying computations the choice of basis vectors is restrictedherein. It will be understood by those skilled in the art that thebinary Walsh code vector w_(o) is obtained by setting each c_(i) to zeroand that w_(j)+w_(j)=w_(o) for any code vector w_(j).

[0021] In order to obtain the parameters c_(i) for any Walsh code w_(j)the integer j in its binary form is represented as:

j=c ₁+2_(c2)+4_(c4)+8_(c8,)+16_(c16)+32_(c32).

[0022] Using this representation:

w _(j) =c ₁ w ₁ +c ₂ w ₂ +c ₄ w ₄ +c ₈ w _(8,) +c ₁₆ w ₁₆ +c ₃₂ w ₃₂.

[0023] The code parameters are referred to as the components of thebinary Walsh code vector w_(i). Furthermore, in order to obtain theWalsh code index of the sum of any two Walsh code vectors w_(i) andw_(j) the components of w_(i) and w_(j) are obtained. If {c₁, c₂ , c₄,c₈, c₁₆, c₃₂} and {c′₁, c′₂, c′₄, c′₈ , c′₁₆, c′₃₂} are the respectivecomponents of vectors w_(i) and w_(j) and k is the index of the sum,then:

k=(c ₁ ⊕c′ ₁)+2(c ₂ ⊕c′ ₂)+4(c₄ ⊕c′ ₄)+8(c₈ ⊕c′ ₈)+16(c₁₆ ⊕c′ ₁₆)+32(c₃₂⊕c′ ₃₂)

[0024] where ⊕ D denotes modulo 2 addition.

[0025] It will also be understood that addition modulo 2 of binary Walshcode vectors is equivalent to multiplication of the corresponding Walshsignals. Thus, W_(i)·W_(j) is also a Walsh signal and it corresponds tothe binary Walsh code vector w_(i)+w_(j). Hereinbelow, the dot productof Walsh signals W_(i), W_(j) is represented as W_(<i,j>).

[0026] Another important feature of Walsh code vectors is their maximumrun length. The maximum run length of a binary Walsh code vector is themaximum number of continuous zeros or ones in the code vector. Oneproperty of the ordering of Walsh code vectors that if the Walsh codeindex is a multiple of eight the maximum run length of the vector is amultiple of sixteen. The one exception is w₈ which has a maximum runlength of eight.

[0027] Another property of the ordering of Walsh code vectors is thatthe maximum run length of the vector is either four or eight if theWalsh code index is a multiple of four but not a multiple of eight. Allother Walsh codes have a maximum run length of four or less. Thus theWalsh codes with the largest maximum run lengths are the ones with anindex that is multiple of eight.

[0028]FIG. 1 shows a block diagram representation of waveform generationsystem. Within waveform generation system summing circuit 106 receivesand combines a plurality of input signals 102A-N. Each input signal 102i is formed of a traffic channel gain {G_(i)}, a message symbol {D_(i)},and a Walsh code bit W_(i). The traffic channel gain {G_(i)} can be zerofor inactive channels. For convenience all digital signals and waveformsare represented for a single symbol period i.

[0029] The waveform output of waveform generation system includes anin-phase component I(t) and an out-of-phase component Q(t). The outputof system is applied to a high power amplifier (not shown) fortransmission within the communications system of the present invention.The output of waveform generation system can be expressed as:

r(t)=I(t)cos(2πf _(c) t)−Q(t)sin(2πf _(c) t)

[0030] The envelope of the signal r(t) is:

A(t)={square root}{square root over (I ²(t)+Q ²(t))}.

[0031] The output signal of waveform generation system can also beexpressed as:${I(t)} = {\sum\limits_{i,n}{G_{i}d_{i}a_{n}{W_{i}\lbrack n\rbrack}{h( {t - {nT}} )}}}$${Q(t)} = {\sum\limits_{i,n}{G_{i}d_{i}b_{n}{W_{i}\lbrack n\rbrack}{h( {t - {nT}} )}}}$

[0032] where T is the chip interval. Combining the above representationsproduces: $\begin{matrix}{{A^{2}(t)} = {\sum\limits_{i_{1},i_{2}}{\sum\limits_{n_{1},n_{2}}{G_{i_{1}}G_{i_{2}}d_{i_{1}}{d_{i_{2}}( {{a_{n_{1}}a_{n_{2}}} + b_{n_{1}b_{n_{2}}}} )}{W_{i_{1}}\lbrack n_{1} \rbrack}{W_{t_{2}}\lbrack n_{2} \rbrack}{h( {t - {n_{1}T}} )}{{h( {t - {n_{2}T}} )}.}}}}} & \text{Eqn.~~(1)}\end{matrix}$

[0033] h(t−n₁T)h(t−n₂ T) is very small when |n₁−n₂|≧2. Additionally,h(t−n₁T)h(t−n₂T) is relatively insensitive to which Walsh code vectorsare used. Thus, the envelope squared A²(t) set forth in Eqn. (1) can beexpressed as a sum of three terms as follows: $\begin{matrix}{{A^{2}(t)} \approx {{\sum\limits_{i_{1},i_{2}}{\sum\limits_{n_{1}}{G_{i_{1}}G_{i_{2}}d_{i_{1}}d_{i_{2}}{W_{{< i_{1}},{i_{2} >}}\lbrack n_{1} \rbrack}{h( {t - {n_{1}T}} )} \quad )^{2}}}} + {\sum\limits_{i_{1},i_{2}}{\sum\limits_{n_{i}}{G_{i_{1}}G_{i_{2}}d_{i_{1}}d_{i_{2}}{W_{{< i_{1}},{i_{2} >}}\lbrack n_{1} \rbrack}{W_{i_{1}\quad}\lbrack n_{1} \rbrack}{W_{i_{1}}\lbrack {n_{1} + 1} \rbrack}{h( {t - {n_{1}T}} )}{h( {t - {nT1}} )}}}} + {\sum\limits_{i_{1},i_{2}}{\sum\limits_{n_{i}}{G_{i_{1}}G_{i_{2}}d_{i_{1}}d_{i_{2}}{W_{{< i_{1}},{i_{2} >}}\lbrack n_{1} \rbrack}{W_{i_{1}}\lbrack n_{1} \rbrack}{W_{i_{1}}\lbrack {n_{1} + 1} \rbrack}{h( {t - {n_{1}T}} )}{h( {t - {n_{1}T} + T} )}}}}}} & \text{Eqn.~~(2)}\end{matrix}$

[0034] where W_(<i) _(1,) _(i) ₂ _(>) is the Walsh code word that is thecomponent-wise product of the vector sets W_(i) and W_(j).

[0035] The first term on the right side of Eqn. (2) is by far thedominant term. It does not depend upon which Walsh code is assigned. Itdepends only upon the product of each pair of assigned Walsh codes or,in the binary domain, upon the sum of the assigned Walsh codes. Theprobability that the first term is large is much higher when severalWalsh codes W_(<i) _(1,) _(i) ₂ _(>) within the first term have a largerun length. This occurs especially when the product or sum of each pairof assigned codes is a multiple of eight. It is a property of thestandard indexing of Walsh code vectors that vectors having indices thatare a multiple of eight have run lengths that are a multiple of eight.

[0036] If W_(<i) _(1,) _(i) ₂ _(>) is the same Walsh code vector forseveral pairs of assigned Walsh codes the first term of Eqn. (2) tendsto be large. At chip sampling times the second and third terms of Eqn.(2) vanish if h(t) is a Nyquist filter. Thus, is likely that more peaksoccur at times other than the chip sampling times. The second and thirdterms Eqn. (2) are relatively insensitive to the specific Walsh codes.The peak to average ratio also depends on the traffic channel gains.Furthermore, according to Eqn. (2) the peak to average ratio tends to bemaximum when the active channel gains are approximately equal. As theactive channel gains become greatly uneven, the peak to average rationtends to reduce somewhat.

[0037] Thus, based upon the foregoing, it appears that the primarydeterminant of the peak to average ratio in not the particular Walshcodes assigned. Rather, it appears that the primary determinant is therun length of the product of each pair of Walsh codes. Furthermore, itappears that the peak to average ratio is high if Walsh codes withindices that are a multiple of eight appear often in products of pairsof assigned Walsh codes.

[0038]FIG. 2 shows a block diagram representation of waveform generationsystem 200. Within waveform generation system 200 summing circuit 206receives and combines a plurality of input signals 202A-N. Each inputsignal 202 i is formed of a traffic channel gain {G_(i)}, a messagesymbol {d_(i)}, and a Walsh code bit W_(i). The traffic channelgain{G_(i)} can be zero for inactive channels. For convenience alldigital signals and waveforms are represented for a single symbol periodi.

[0039] In accordance with the method of the present invention permutingoccurs after the combining that is performed by summing circuit 100.Thus the output of summing circuit 106 is applied to permuting block toperform the permuting operations described herein. The permuted outputof permuting block is applied to mixers 114, 124 for mixing with signals210, 220, respectively.

[0040] The waveform output of waveform generation system thus includesan in-phase component I(t) at the output of transform 216 and anout-of-phase component Q(t) at the output of transform 226. The outputof system 200 is applied to a high power amplifier (not shown) fortransmission within the communications system of the present invention.

[0041]FIG. 3 shows graphical representation 300 that compares the peakto average ratio at equal channel gains and the peak to average ratio athighly unequal channel gains. Graphical representation 300 is a 1-CDFwherein N=8, RS 2 fixed full rate, where CDF is a cumulativedistribution function In order to make this comparison a simulation isperformed with a plurality of Walsh code sets using only the Walsh codesset forth hereinbelow. No overhead channels are considered. Furthermore,all traffic channels are assumed to have the same traffic channel gain.

[0042] The following three sets of Walsh codes are used in thesimulation of graphical representation 300. Each of the three setscontains eight Walsh codes. (1) WCC-1={1, 9, 17, 25, 33, 41, 49, 57}.Thus, in accordance with WCC-1, the product of every pair of Walsh codesused in the simulation has an index that is a multiple of eight. (2)WCC-2={0, 1, 2, 4, 8, 9, 10, 12}. Thus, a moderate number of pairs ofWalsh codes used in the simulation have a binary sum with an index thatis a multiple of eight. (3) WCC-3={0, 1, 2, 3, 4, 5, 6, 7}. Thus, nopair of Walsh codes set forth in graphical representation 300 has abinary sum with an index that is a multiple of eight. Thus, FIG. 3illustrates that the peak to average obtained using WCC-1 is much higherthan the peak to average obtained using WCC-2. WCC-2, in turn, providesa peak to average that is higher than the peak to average obtained usingWCC-3. These results are consistent with the results set forth above.

[0043] Thus four rules are provided for approximately determining thepeak to average ratio properties of a set of Walsh codes of fixed sizeas follows. Rule I is directed to sets of codes having a higher numberof pairs of binary Walsh code vectors whose modulo 2 sum is a Walsh codewith an index that is a multiple of eight. Such sets of code vectors areexpected to have a higher peak to average ratio.

[0044] In accordance with Rule I, Rule II is directed to cases whereinthere is a higher frequency of occurrence of each Walsh code as theproduct of pairs of Walsh codes in the set. These cases also correspondto a higher peak to average ratio. Based upon the simulations it isbelieved that Rule I is more important than Rule II.

[0045] Rule IV is directed to cases wherein there is (a) a higherproduct of traffic channel gains for a pair of binary Walsh codevectors, and (b) the sum of the binary Walsh code vectors gives a Walshcode with an index that is a multiple of eight. In such cases the pairof binary Walsh code vectors makes a higher contribution to the peak toaverage ratio other pairs. For example, the pilot channel is assignedWalsh code zero and a high channel gain. A traffic channel can then beassigned a Walsh code with an index that is a multiple of eight. Thecontribution of this pair of codes to the peak to average ratio is moresignificant For example, the contribution of this pair is moresignificant than the contribution of a pair of Walsh codes with a binarysum that is a Walsh code with an index that is a multiple of eight.

[0046]FIG. 4 show graphical representation 400 for comparing the peak toaverage ratio in the case of equal channel gain along with highlyunequal channel gain. Graphical representation 400 is a 1-CDF plot withdifferent transmit channel gains. The Walsh code set used in thesimulation of graphical representation 400 is WCC-2. As previouslydescribed with respect to graphical representation 300, N=8, RS2, andfixed full rate. Thus, graphical representation 400 illustrates that ifthe traffic channel gains are approximately equal the peak to averagetends to be higher than if they are unequal.

[0047] FIGS. 5A-E show a block diagram representation of biased binbalancing algorithm 500. When Walsh codes are assigned to new callswithin a communications system according biased bin balancing algorithm500, the occurrence of large peaks in the transmit power level of thecombined signals is reduced to one tenth of a percent. This should becompared with an occurrence of approximately one percent using a randomWalsh code assignment method.

[0048] Computations involving Walsh codes performed during the executionof biased bin balancing algorithm 500 can be performed using theteachings set forth herein. The execution of biased bin balancingalgorithm 500 can be performed in a call resources database managementunit at the base station. A structure useful in performing biased binbalancing algorithm 500 is referred to as a Walsh code control block.This data structure can be maintained at the base station.

[0049] Biased bin balancing algorithm 500 is based primarily upon RuleI. It is also partially based upon Rule III. The significance of Rule IIis believed to be in providing a possible alternate embodiment.Furthermore, Rule IV is believed to be primarily related to the casewherein the pilot channel gain is higher than the channel gain ofremaining channels. Biased bin balancing algorithm 500 is adapted toassign a Walsh code to a new user, either an originating call or ahandoff, of a communication system in a way that the binary sum of theWalsh code with the minimum possible currently active Walsh codes has anindex that is a multiple of eight and to partially incorporate Rule II.In accordance with the method of the present invention a new call isassigned a Walsh code immediately upon request provided that theresources for doing so are available. Updates to the Walsh code controlblock are carried out immediately after a user is assigned or unassigneda Walsh code.

[0050] The Walsh code control block contains bins, or data structuresυ₀, υ₁, υ₂, υ₃, υ₄, υ₅, υ₆, υ₇ for storing information about Walshcodes. A Walsh code belongs to the bin υ_(i) if its index modulo 8 is i.Furthermore, a Walsh code is said to be active in the bin υ_(i) if itbelongs to bin υ_(i) and is currently assigned to an active trafficchannel or an overhead channel. Otherwise, the Walsh code is said to beinactive. Each bin υ_(i) stores an indication of each Walsh code thatbelongs to it, including Walsh codes assigned to traffic channels and tooverhead channels. The total number of active Walsh codes in the binυ_(τ) is represented as bin_value.

[0051] For example, the bin_value of a bin υ₁ can include the number ofassigned Walsh code indices from the set {w₂, w₁₀, w₁₈, w₂₆, w₃₄, w₄₂,w₅₀, w₅₈}. The bin capacity, or maximum bin_value, of bin vi istherefore eight. The bin capacity can be verified by noting that themodulo 2 sum of any two binary Walsh code vectors belonging to the samebin is a multiple of eight and that the sum of any two binary Walsh codevectors belonging to different bins is not a multiple of eight. Thus,the bin υ_(i) has the bin_label that is equal to i.

[0052] The Walsh code control block also contains an integer variablecycle. Each Walsh code with indices 8i through 8i+7 is defined to havecycle i. Thus, with a Walsh code size of sixty-four, the values of cycleare between zero and seven. A Walsh code is uniquely determined byspecifying its cycle and its bin_label values. The Walsh code controlblock contains an integer array WC_assign which is of the form:

WC_assign=[current_cycle,current_bin_label]

[0053] wherein current_cycle is of the type cycle, and current_bin_labelis of the type bin_label. The array WC_assign points to the cycle andbin_label of the Walsh code that is currently available for assignmentto the next call request.

[0054] In the initial state of the method of the present invention thereare no traffic channels. In this state the bin ν₀ includes only thepilot channel and the synchronization channel and therefore for bin ν₀bin_value=2. Additionally, the bin υ₁ includes only the paging channeland therefore for bin υ₁ bin_value=1. All other bins are set withbin_value=0. Additionally, current_cycle is set to zero andcurrent_bin_label is set to two.

[0055]FIG. 6 shows Walsh code assignment state diagram 600. Walsh codeassignment state diagram 600 represents a process performed inaccordance with the present invention and includes a total of fourstates. Transition from idle state 610 of assignment state diagram 600is controlled by two binary variables, new_user_arrives andold_user_departs. The two binary variables are set to a value of TRUEwhen a new user, either an originating or a handoff, requests a Walshcode channel or an old user is unassigned a Walsh code channel,respectively.

[0056] When new_user_arrives becomes TRUE the process of state diagram600 leaves idle state 610 and enters assign Walsh code update bin state620. In state 620 the Walsh code referenced by the current value ofWC_assign is assigned to the user making the request. The assigned Walshcode is set to active in the bin having the label current_bin_label. Thebin_value of the bin with the label current_bin_label is incremented. Astate transition from state 620 to update_ptr state 640 then occurswithin state diagram 600.

[0057] When old_user_departs becomes TRUE the process of state diagram600 leaves idle state 610 and enters unassign Walsh code update binstate 630. In state 630 the Walsh code of the departing user isunassigned. The unassigned Walsh code is set to inactive in the binwhere it had been previously been assigned. The bin_value of the bin isdecremented. A state transition from state 630 to update_ptr state 640then occurs within state diagram 600.

[0058] Biased bin balancing algorithm 500 sets forth the operationsperformed in accordance with the present invention within update_ptrstate 640 of Walsh code assignment state diagram 600. In one preferredembodiment of the invention the bins are loaded uniformly. This providesa substantial improvement over random assignment of the Walsh codes withrespect to peak to average ratio. However, further improved performancecan be obtained by biasing the loading of the bins somewhat. Forexample, it is preferred to give the least preference to the bin υ₀because the bin υ₀ carries the pilot signal which has a high gain.Furthermore, the bin υ₁ receives less preference than bins υ₂ through υ₇because the bin υ₁ contains the paging channel. The remaining bins υ₂through υ₇ receive equal preference.

[0059] In biased bin balancing algorithm 500 a determination is madewhich of the bins between bin υ₀ and bin υ₇ contain the minimum numberof assigned active Walsh codes as shown in block 505. For example, ifbins υ₂ and υ₃ contained three active Walsh codes and the remainder ofthe bins contained more than three, the operations of block 505 wouldreturn bins υ₂ and υ₃. The bins containing the minimum number ofassigned active Walsh codes in the subset of bins consisting of bins υ₂through υ₇ is then determined as shown in block 510.

[0060] Execution of biased bin balancing algorithm 500 then proceeds todecision 515 where a determination is made with respect to the number nof bins between bins υ₂ and υ₇ that was determined in block 510. If n==1execution of biased bin balancing algorithm 500 proceeds from FIG. 5A toFIG. 5B by way of off-page connector 519 and on-page connector 521.Since only a single bin has the minimum number of active codes in thiscase, biased bin balancing algorithm 500 merely selects one of theavailable inactive codes in the single bin as shown in block 520. Asshown in block 525 the selected code is assigned to the new call. Thecurrent cycle is incremented as shown in block 530 and execution ofbalancing algorithm 500 proceeds to exit 535. If more than one binbetween bin υ₂ and bin υ₇ has the minimum number of active Walsh codes,as determined by decision 515, execution of biased bin balancingalgorithm 500 proceeds from FIG. 5A to FIG. 5C by way of off-pageconnector 518 and on-page connector 551.

[0061] When this path is taken algorithm 500 attempts to assign thelowest index Walsh code that is available in one of the bins with theminimum number of active codes as shown in block 550. Thus the Walshcodes can be divided, for example, into eight consecutive cyclesaccording to their index modulo 8. Only if no inactive Walsh codes areavailable within the cycle indicated by the current-cycle pointer is aWalsh code from the next cycle used as shown in block 560. The selectedWalsh code is then assigned to the new call as shown in block 570.Execution then exits by way of exit terminal 575.

[0062] If the bin υ₁ has the minimum number of active Walsh codes, asdetermined by decision 515, execution of biased bin balancing algorithm500 proceeds to FIG. 5D by way of off-page connector 517 and on-pageconnector 581. The path of off-page connector 517 is not taken unlessneither of the paths of off-page connectors 518, 519 is taken. In thismanner algorithm 500 is biased against bin vi as previously described.An inactive Walsh code is located in bin Vi as shown in block 580. Thelocated Walsh code is assigned to the new call as shown in block 585. Inblock 590 the current cycle is incremented and execution proceeds toexit 595.

[0063] When execution of biased bin balancing algorithm does not proceedfrom decision 515 by way of any of the other paths, it proceeds to FIG.5E by way of off-page connector 516 and on-page connector 820. Aninactive code is then found in the current bin as shown in block 830 andassigned to the new call as shown in block 840. The current cycle isincremented as shown in block 850 and execution exits algorithm 500 byway of exit 860.

[0064] A more detailed description of biased bin balancing algorithm 500is set forth in Table I. The representation of Table I is a conventionalpseudocode representation understood by those skilled in the art.Compute the set min_bin = {bins with minimum bin value}. Compute the setmin_bin_sub = {υ_(i) 1 υ_(i) ε min_bin,2≦i≦7}. If (1 min_bin_sub 1 == 1)For (i=0; ++i;i<8) If (Walsh code at [min_bin_sub_label,current_cycle+i]is inactive) Set WC_assign = [current_cycle+i, min_bin_sub_label]; setcurrent_cycle = current_cycle +i; exit; Elseif (1 min_bin_sub 1 > 1) For(i=0; ++i;i<8) {Set current_cycle = min{i 1ww at[min_bin_sub_,current_cycle+i] is inactive} Set current_bin_label =min{bin_label 1 corresp. bin ε w at min_bin_sub,WC at[current_cycle,bin_label] is inactive} Set WC_assign =[current_cycle,current_bin_label];} Elseif (υ_(i)ε min_bin) For(i=0;++i; i<8) If (Walsh code at [current_cycle+i,1] is inactive); SetWC_assign = [current_cycle+i,1]; set current_cycle = current_cycle +i;exit; Else For (i=0;++i; i<8) If (Walsh code at [current_cycle+i,0] isinactive); Set WC_assign = [current_cycle+i,0]; set current_cycle =current_cycle +i; exit;

[0065] In an example of the operation of biased bin balancing algorithm500 only the pilot, paging and sync channels are active. They have Walshcodes 0, 1 and 32 respectively. Thus, bins υ₀ and υ₁ have a bin_value oftwo and one, respectively, and all other bins have bin_value=0. Althoughit is believed that the primary benefit of the system of the presentoccurs in a dynamic communication system, an assumption is made thateach new call requiring a Walsh code is active for a long period oftime. Thus, in this example, once a Walsh code is assigned it is notunassigned. Under these conditions, biased bin balancing algorithm 500provides the following sequence:2,4,5,6,7,10,11,12,13,14,15,9,18,19,20,21,22,23,17,8,26,27, . . .

[0066] Two principles upon which biased bin balancing algorithm 500 isbased are as follows. First, different Walsh code assignmentscorresponding to any single bin configuration exhibit approximately thesame peak to average ratio. The bin values corresponding to specificWalsh code assignment can be referred to as the bin configuration of thecorresponding set of assigned Walsh codes. Secondly, as the imbalance inthe bins in a bin configuration increases the peak to average ratioincreases.

[0067] In order to test the first principle the bin configuration ofTable II is provided. In this configuration there are eight activetraffic channels. The pilot, paging and synchronization channels areassigned Walsh codes 0, 1, 32, respectively. TABLE II υ₀ υ₁ υ₂ υ₃ υ₄ υ₅υ₆ υ₇ 2 1 2 2 1 1 1 1

[0068]FIG. 7 shows graphical representation 900 that compares the peakto average ratio for different Wash code assignments with the same binconfiguration as shown in Table II. Graphical representation 900 is a1-CDF plot, N=8, RS2, fixed full rate. The Walsh code assignments forthe active traffic channels are:

[0069] WCC-1={2,3,4,5,6,7,10,11}

[0070] WCC-2={2,3,12,13,22,23,42,43}

[0071] WCC-3={2,11,20,29,38,47,58,3}

[0072] WCC-4={2,3,4,5,38,39,42,43}.

[0073] The peak to average ratios for the Walsh codes of graphicalrepresentation 900 are very close together. The slight increase in thepeak to average ratio of WCC-3 is believed to be related to Rule IIIabove.

[0074] In order to test the second principle set forth above a case isconsidered wherein there are fourteen active traffic channels and thepilot, paging, and synchronization channels are assigned Walsh codes 0,1, 32, respectively. A series of Walsh code sets and bin configurationsare set forth below as Table III. The WCC-1 case is substantiallybalanced. The imbalance is increased in WCC-2 case and the imbalance inthe WCC-3 case is further increased. The imbalance in the WCC-4 case ismaximum.

[0075]FIG. 8 shows graphical representation 1000 illustrating the peakto average ratio of the Walsh code sets of Table III. In accordance withthe waveforms of graphical representation 1000 the peak to averageratios of the Walsh code sets increase as the bin imbalance increases.

[0076] Further to Rule IV above, the peak to average ratio also dependson the traffic channel gains. Thus, in an alternate embodiment of biasedbin balancing algorithm 500 the bin_value of each bin can contain thetraffic channel gains corresponding to the Walsh codes belonging to thebin. Two methods of updating the bin values using the traffic channelgains can be used in accordance with this alternate embodiment in orderto improve performance.

[0077] One method is to use the traffic channel gains only immediatelyafter assigning or unassigning Walsh codes and accordingly setWC_assign. The other method is to periodically update the bins andaccordingly set WC_assign. The latter method can yield somewhat improvedperformance, since the traffic channel gains vary dynamically duringsystem operation. However, this results in increased complexity. Thestate update_ptr of update pointer state diagram 600 is not affected bythis alternate embodiment.

[0078] In a further alternate embodiment the probability of severalpairs of Walsh summing up to give the same Walsh code or codes islimited. This alternate embodiment is provided in accordance with RuleII above. In this embodiment only the state update_ptr is affected. Toperform algorithm 500 in accordance with this embodiment, the variablecurrent_cycle is incremented by at least one after every code channelassignment within the update_WC_assign( ) procedure. Thus the openingloop statement For (i=0;++i; i<8) in Table ______ is changed to For(i=1;++i; I<8).

[0079] When blocks of code channels are assigned at the same timemodifications can be made to biased bin balancing algorithm 500. Blockassignments can occur, for example, when multiple data rate issupported, where multiple data rate refiers to the assignment ofmultiple code channels at the same time. The array WC_assign can be ofthe form:

WC_assign={current cycle₂, current_bin_label₁, . . . current_cycle_(M),current_bin_label_(M)}

[0080] where M is the maximum number of code channels that can beassigned at a time.

[0081] A further alternate embodiment is provided by the fact Walsh codewords having an index that is a multiple of four also have a relativelylarger run length, for example four or eight. If multiple bins have theminimum bin_value a Walsh code can be assigned from the bin with auseful property. A minimum number of active Walsh code words form amodule 2 sum with the active Walsh code words of such a bin giving Walshcode words with indices that are a multiple of four. In order to providethis embodiment each bin υ_(j) stores the sum of bin_values υ_((j+4))mod 8 and υ_((j−4)) mod 8.

[0082] In order to estimate the effectiveness of biased bin balancingalgorithm 500 a formula is provided. The formula is useful inapproximating the probability of various unbalanced bin configurationsoccurring during random Walsh code assignments. As previously described,when bin imbalance occurs the peak to average ratio increases.

[0083] In this formula the total number of active and overhead trafficchannels is represented as N. When the Walsh codes for all the N codechannels are assigned randomly there is a probability that at least jbins contain at least M assigned Walsh codes each can be shown to be:${P( {N,M,J} )} = {{\min \lbrack {1,\frac{\begin{pmatrix}8 \\J\end{pmatrix}\quad \begin{pmatrix}8 \\M\end{pmatrix}^{J}\begin{pmatrix}{64 - {JM}} \\{N - {JM}}\end{pmatrix}}{\begin{matrix}64 \\N\end{matrix}}} \rbrack}.}$

[0084] The approximation of this probability is very good for largevalues of M.

[0085]FIG. 9 shows graphical representation 1100, illustrating aprobability plot for bin imbalance. The imbalance in a bin configurationif most of the assigned Walsh codes belong to a small number of bins.This corresponds to a large value of M and, possibly, a large value ofj. Graphical representation 1100 shows the probability P(N, M, J) forvalues M and J where N=17. In another alternate embodiment, bins v₃ andv₄ can be selected randomly if they both have the same number of activeWalsh codes.

[0086] The previous description of the preferred embodiments is providedto enable a person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features is disclosed.

1. A method of transmitting message signals in a communications systemhaving a new call entering the communications system and Walsh codesthat can be active and inactive, comprising of the steps of: (a)dividing the Walsh codes into bins; (b) determining the number of activeWalsh codes in the bins; (c) selecting a Walsh code in accordance withthe determination of step (b); and (d) assigning the selected Walsh codeto the new call. 2 An apparatus for transmitting message signals in acommunications system having a new call entering the communicationssystem and Walsh codes that can be active and inactive, comprising: (a)bins for dividing the Walsh codes into the bins; (b) a valuerepresentative of the number of active Walsh codes in the bins; (c) aselected Walsh code determined in accordance with the value; and (d) anew call for assigning the selected call to the Walsh code. 3 Anapparatus for transmitting message signals in a communications systemhaving a new call entering the communications system and Walsh codesthat can be active and inactive, comprising: (a) means for dividing theWalsh codes into bins; (b) means for determining the number of activeWalsh codes in the bins; (c) means for selecting a Walsh code inaccordance with the determination of step (b); and (d) means forassigning the selected Walsh code to the new call.